1. Field of the Invention
The present invention relates to an optical deflection element, and more particularly to an optical waveguide element capable of two-dimensionally deflecting a light beam incident to an optical waveguide by electro-optic effect. The optical deflection element is applicable to the entire spectrum of optoelectronics including laser printer, digital copier, facsimile, display, optical interconnection, optical crossconnect, bar-code reader, glyph code reader, optical disk pickup, optical scanner for surface inspection, optical scanner for surface shape presumption, and the like.
2. Description of the Prior Art
A typical laser beam optical deflector used in a laser beam printer, digital copier, facsimile, and the like has a rotating polygonal mirror called a polygon mirror for deflecting a beam from a gas laser and a semiconductor laser and an fxcex8 lens for converging the laser beam reflected by the rotating polygonal mirror to the state of line motion of a uniform rate on an imaging surface such as a photoreceptor. Such an optical deflector employing a polygon mirror is large in size, and has problems in that it is lacking in durability and generates noise because the polygon mirror is mechanically, fast rotated by a motor, and a light scanning speed is limited by the number of rotations of the motor. A galvanomirror and a cantilever mirror are problematic in terms of deflection precision.
On the other hand, optical deflection elements taking advantage of acoustooptical effect are available as solid, electrical laser beam optical deflectors. Of them, optical waveguide elements smaller than bulk acoustooptical elements are especially expected. The optical waveguide elements are under study of application to a printer or the like as laser beam optical deflection elements to solve the drawbacks of laser beam optical deflectors employing a polygon mirror. The optical deflection elements of optical waveguide type have: an optical waveguide constructed from LiNbO3 and ZnO; a unit for making a laser light beam incident to the optical waveguide; comb electrodes for pumping surface acoustic waves for deflecting the light beam within the optical waveguide by acoustooptical effect; and a unit for emitting the deflected light beam from the optical waveguide. Additionally, as required, thin-film lenses and the like are added to the elements. However, optical deflection elements taking advantage of acoustooptical effect generally have the following problems: the upper limit of laser deflection speed due to sonic limitation; reduced light utilization efficiency and processing of zero-order light because of diffraction efficiency as low as several tens of percent; and an expensive and large power supply unit for controlling frequencies of several hundred MHz. For this reason, it has been difficult to apply such optical deflection elements to laser printer, digital copier, facsimile, display, optical interconnection, bar-code reader, glyph code reader, optical disk pickup, and the like.
On the other hand, there are known prismatic optical deflection elements, as described in the literature A. Yariv, Optical Electronics, 4th ed. (New York, Rinehart and Winston, 1991) pp. 336 to 339, that employ oxide ferroelectric materials having electro-optic effect higher in modulation speed than acoustooptical effect. Although bulk elements employing ceramics or monocrystals are available as the prismatic optical deflection elements, they have been incapable of providing a practical deflection angle because of its large size and a substantially high driving voltage. Prismatic domain inversion optical deflection elements or prismatic electrode optical deflection elements are described in the literature Q. Chen, et al., J. Lightwave Tech. vol. 12 (1994) 1401. and Japanese Published Unexamined Patent Application No. Hei 1-248141. The prismatic domain inversion optical deflection elements or prismatic electrode optical deflection elements have prisms disposed in cascaded form by using a LiNbO3 monocrystalline wafer on which a Ti diffused optical waveguide or proton exchanged optical waveguide is formed. An electrode separation of approximately 0.5 mm, which is equal to the thickness of a LiNbO3 monocrystalline wafer, is required. Therefore, there exist problems in that a driving voltage is still high and a deflection angle as small as approximately 0.2 degrees obtained with a driving voltage of xc2x1600 V, as described in the above-described literature, is far from a practical level. There is disclosed in Japanese Published Unexamined Patent Application No. Hei 2-311827 a method which changes the effective refractive index of an optical waveguide by acoustooptical effect and changes emission angles from a prism coupler. However, with the disclosed configuration, electrodes are not disposed so as to change the refractive index of an optical waveguide portion in which a prism coupler is disposed. Accordingly, there exists a problem in that emission angles cannot actually be changed, and even if emission angles could be changed, because of the construction that electrodes are disposed on the optical waveguide surface, the electrode interval would increase and a practical deflection angle could not be obtained as in the example of the above-described literature.
On the other hand, the inventor proposed (Japanese Published Unexamined Patent Application No. Hei 9-5797) a prismatic deflection element to solve the above-described driving voltage problem wherein the prismatic deflection element has an oxide optical waveguide having epitaxial or single orientation with electro-optic effect and a light source to make a light beam incident to the optical waveguide, and employs a thin-film optical waveguide provided with electrodes for deflecting the light beam within the optical waveguide by electro-optic effect. However, a distribution of electric field of a laser beam propagating through an optical waveguide s penetrates into a substrate. The absorption coefficient of a substrate having a practical resistivity is often large, penetrationg components are strongly absorbed by free carriers in a conductive substrate, and propagation loss in a thin-film optical waveguide reaches tens of dB/cm due to the absorption in addition to loss due to scattering of the optical waveguide itself, raising a problem that light utilization efficiency is practically insufficient.
Generally, in elements having a coplanar electrode placement, an SiO2 clad layer is inserted between a metal electrode on an optical waveguide and the optical waveguide to prevent the penetration of electric fields into the metal electrode and avoid absorption of propagating light. However, there is a problem in that, if SiO2 were provided between a conductive substrate and an oxide optical waveguide, an oxide optical waveguide having epitaxial or single orientation with electro-optic effect could not be fabricated because SiO2 is amorphous. Moreover, there is a problem in that the relative dielectric constant of an oxide optical waveguide material having electro-optic effect ranges from several tens to several thousands, which is very large compared with the relative dielectric constant 3.9 of SiO2, and since series capacitors are formed as an equivalent circuit in the described-above construction of a thin-film waveguide on a conductive substrate, an effective voltage applied to the thin-film optical waveguide is no more than several percent of an applied voltage, ultimately causing a significant increase in a driving voltage. In an i-GaAs waveguide of a compound semiconductor, an i-AlGaAs clad layer is inserted between the i-GaAs waveguide and n-AlGaAs lower clad layer to prevent the penetration of electric fields into the n-AlGaAs lower clad layer, thereby avoiding absorption by free carriers of the n-AlGaAs lower clad layer. However, a method for providing the same construction for an oxide optical waveguide has not been known wherein the oxide optical waveguide is made of a material entirely different from a compound semiconductor, is difficult of epitaxial growth, and has an electro-optic effect of relative dielectric constants ranging from several tens to several thousands. For this reason, the inventor has proposed a construction that an epitaxial or single orientation with buffer layer of a high relative dielectric constant is provided on a conductive substrate, on top of it is provided an oxide thin-film optical waveguide having epitaxial or single orientation with electro-optic effect, and further on top of it is provided an electrode, making it possible to satisfy both low driving voltage characteristics and low light propagation loss characteristics.
However, there is a problem in that both the above described optical deflection elements only perform one dimensional light deflection, and since two-dimensional deflection requires a combination of two deflection units, at least adjustments of an optical axis are required or apparatus construction is complicated. For example, to perform two-dimensional deflection by using the method disclosed in Japanese Published Unexamined Patent Application No. Hei 9-5797, combinations with a polygon mirror, a galvanomirror, a cantilever mirror, a bulk acoustooptical element, or a bulk electro-optic element are considered. However, in any event, the conventional problems will occur. Accordingly, two-dimensional deflection must be performed by one optical waveguide element.
As such a two-dimensional deflection element, in Japanese Published Unexamined Patent Application No. Sho 62-238537, an optical waveguide element capable of two-dimensional deflection is disclosed. The optical waveguide element has two units for providing acoustooptical effect by surface acoustic waves to change each of deflection angles within a substrate and emission angles in an emission part. However, the following problems remain still unsolved: the upper limit of laser deflection speeds due to the sonic limitation of acoustooptical deflection; reduced light utilization efficiency and the need to process plural-order light because of diffraction efficiency as small as several tens of percent; and an expensive and large-scale driving circuit for controlling frequencies of several hundreds of MHz. There is disclosed in Japanese Published Unexamined Patent Application No. Sho 58-130327 a two-dimensional deflection element that performs deflection within a substrate by acoustooptical effect of surface acoustic waves and changes emission angles by electro-optic effect by applying a voltage to an emission grating part. However, there still remain the problems of the upper limit of laser deflection speeds due to the sonic limitation of acoustooptical deflection and an expensive and large-scale power supply unit for controlling frequencies of several hundreds of MHz. Since an electrode for controlling the refractive index of the grating part is disposed on a optical waveguide surface, the interval between electrodes increases, with the result that application of 100 V causes an emission angle change of approximately 0.04 mrad (0.002 degrees), which is far from practical deflection angles as in the examples of the above-described literature and official gazettes. Also, since diffraction must be finally used also at the grating part, light utilization efficiency decreases or zero-order light and plural-order light must be processed. Furthermore, there arises a new problem that the use of acoustooptical effect and electro-optic effect requires that the respective driving circuits be used.
The present invention has been made to solve the above-described problems and, by using a single substrate, provides a small-size optical waveguide element that requires no optical axis adjustments, can be driven at a low voltage, is excellent in light utilization efficiency, and can rapidly two-dimensionally deflect a light beam.
The present invention has an optical waveguide having epitaxial or single orientation with electro-optic effect, provided on a conductive or semi-conductive single-crystal substrate to serve as a lower electrode, or a substrate on the surface of which a conductive or semi-conductive single-crystal thin film to serve as a lower electrode is formed, an electrode for controlling a light beam within the optical waveguide, disposed on the optical waveguide, the electrode forming an area, between the electrode and the single-crystal substrate or single-crystal thin film, that changes in refractive index in accordance with an applied voltage and deflects a light beam propagating through the optical waveguide in a first direction in accordance with the applied voltage, an emission prism for emitting the light beam within the optical waveguide in a second direction crossing the first direction, and a transparent electrode for controlling an emitted light beam, disposed between the emission prism and the optical waveguide, the transparent electrode forming an area, between the transparent electrode and the single-crystal substrate or single-crystal thin film, that changes in refractive index in accordance with an applied voltage and deflects a light beam emitted from the emission prism in the second direction in accordance with the applied voltage.
The term xe2x80x9csingle orientationxe2x80x9d refers to the case where, in an X-ray diffraction pattern of a thin film, the strength of a specific crystal plane parallel to a substrate plane is 1% or less with respect to the strength of other crystal faces, and the term xe2x80x9cepitaxialxe2x80x9d refers to the case where a in-plane single orientation thin film also has single-orientation in a face direction of the substrate.
According to the present invention, since an area for deflecting a light beam propagating through an optical waveguide in a first direction in accordance with an applied voltage, and an area for deflecting a light beam emitted from an emission prism in a second direction crossing with the first direction are provided, two-dimensional deflection can be performed by controlling a voltage applied to the electrode for controlling a light beam within the optical waveguide and a transparent electrode for controlling an emitted light beam. Also, since a single single-crystal substrate or a substrate on which a single-crystal thin film is formed is used, there can be provided a small-size optical waveguide element that requires no optical axis adjustments, can be driven at a low voltage, is excellent in light utilization efficiency, and can rapidly two-dimensionally deflect a light beam.
The electrode for controlling a light beam within the optical waveguide of the present invention can be formed in a triangular shape and can have a prism area formed within the optical waveguide wherein the prism area has a different refractive area from surrounding areas upon application of a voltage.
A polarization domain inversion area of prism shape is provided in the optical waveguide, and an electrode for controlling a light beam within the optical waveguide can be formed in a triangular shape, and upon application of a voltage, can form within the optical waveguide the polarization domain inversion area of prism shape having a different refractive index from surrounding areas.
The substrate on which the single-crystal thin film is formed can be formed as a substrate which has a smaller refractive index than the optical waveguide and on the surface of which an epitaxial or single orientation conductive or semi-conductive oxide is provided as a thin film.
The optical waveguide may be provided on the single-crystal substrate or single-crystal thin film through a buffer layer having a smaller refractive index than the optical waveguide.
The conductive or semi-conductive single-crystal substrate can be constructed from a transparent oxide having a smaller refractive index than the optical waveguide.
On the surface of the optical waveguide may be provided a clad layer having a smaller refractive index than the optical waveguide, wherein the optical waveguide can be constructed from an oxide ferroelectric.
In the present invention, the following materials can be used as a conductive or semi-conductive single-crystal substrate to serve as a lower electrode substrate, or a conductive or semi-conductive single-crystal thin film, particularly an epitaxial or single orientation thin film to serve as a lower electrode: SrTiO3 doped with Nb or the like, Al-doped ZnO, In2O3, RuO2, BaPbO3, SrRuO3, YBa2Cu3O7xe2x88x92x, SrVO3, LaNiO3, La0.5Sr0.5CoO3, ZnGa2O4, CdGa2O4, CdGa2O4, Mg2TiO4, MgTi2O4, and other oxides, Si, Ge, diamond, and other single semiconductors, AlAs, AlSb, AlP, GaAs, GaSb, InP, InAs, InSb, AlGaP, AlLnP, AlGaAs, AlInAs, AlAsSb, GaInAs, GaInSb, GaAsSb, InAsSb, and other III-V-family compound semiconductors, ZnS, ZnSe, ZnTe, CaSe, Cdte, HgSe, HgTe, CdS, and other II-VI-family compound semiconductors, Pd, Pt, Al, Au, Ag, and other metals. The use of oxides is often advantageous to the membrane quality of an oxide thin-film optical waveguide disposed in an upper portion. It is desirable that the conductive or semi-conductive single-crystal substrates, or conductive or semi-conductive epitaxial or single orientation thin films are selected in accordance with carrier mobility required by the crystal structure, deflection speed, switching speed, or modulation speed of the ferroelectric thin film. Although it is effective in terms of RC time constant that the resistivity of the thin films is 108 xcexa9xc2x7cm or less, preferably 106 xcexa9xc2x7cm or less, the thin films can be used as a lower electrode if the resistivity is such that voltage drop is negligible. When a silicon substrate is used that, as the refractive index of the thin films, has a large refractive index of, e.g., 3.45, which is higher than the resistivity of normal optical waveguide materials, since the film thickness of the buffer layer must be made considerably thick to prevent the leak of light to the substrate, it is desirable that the thin films have a lower refractive index than the optical waveguide materials to reduce the thickness of the buffer layer and achieve low-voltage driving.
The following materials can be used as a substrate on the surface of which a conductive or semi-conductive epitaxial or single orientation thin film to serve as a lower electrode is formed: SrTiO3, BaTiO3, BaZrO3, LaAlO3, ZrO2, Y2O38%-ZrO2, MgO, MgAl2O4, LiNbO3, LiTaO3, Al2O3, ZnO, and other oxides, Si, Ge, diamond, and other single semiconductors, AlAs, AlSb, AlP, GaAs, GaSb, InP, InAs, InSb, AlGaP, AlLnP, AlGaAs, AlInAs, AlAsSb, GaInAs, GaInSb, GaAsSb, InAsSb, and other III-V-family compound semiconductors, ZnS, ZnSe, ZnTe, CaSe, Cdte, HgSe, HgTe, CdS, and other II-VI-family compound semiconductors. The use of oxides is often advantageous to the membrane quality of an oxide thin-film optical waveguide disposed in an upper portion.
When a buffer layer is used, materials satisfying the following conditions are selected as the material of the buffer layer: the buffer layer has a smaller refractive index than thin-film optical waveguide materials; the ratio of the relative dielectric constant of the buffer layer and that of the optical waveguide is 0.002 or more, preferably the ratio of the relative dielectric constant of the buffer layer and that of the optical waveguide is 0.006 or more; and the relative dielectric constant of the buffer layer is 8 or more. Buffer layer materials must be able to hold an epitaxial relationship with conductive substrate materials and optical waveguide materials. As a condition that the epitaxial relationship can be held, it is desirable that buffer layer materials are similar to conductive substrate materials and optical waveguide materials in crystal structure, and the difference of granting constants is 10% or less. However, this condition need not always be observed if the epitaxial relationship can be held. Specifically, the following materials can be used: tetragonal, orthorhombic, or pseudo-cubic ABO3 perovskite oxides such as SrTiO3, BaTiO3, (Sr1xe2x88x92xBax)TiO3 (0 less than x less than 100), PbTiO3, Pb1xe2x88x92xLax(ZryTi1xe2x88x92y)1xe2x88x92x/4O3 (0 less than x less than 30, 0 less than y less than 100, PZT, PLT, PLZT depending on the values of x and y), Pb (Mg1/3Nb2/3)O3, KNbO3; hexagonal ABO3 perovskite oxides such as ferroelectrics typified by LiNbO3 and LiTaO3; tungsten bronze oxides such as SrxBa1xe2x88x92xNb2O6 and PbxBa1xe2x88x92xNb2O6; and Bi4Ti3O12, Pb2KNb5O15, K3Li2Nb5O15, ZnO, and other substitutive dielectrics. It is effective that the ratio of the film thickness of the clad layer and that of the optical waveguide is 0.1 or more, preferably 0.5 or more, and the film thickness of the clad layer is 10 nm or more.
The optical waveguide is formed in thin film form and thin-film optical waveguide materials are selected from oxides. Specifically, the following materials can be used: tetragonal, orthorhombic, or pseudo-cubic ABO3 perovskite oxides such as BaTiO3, PbTiO3, Pb1xe2x88x92xLax(ZryTi1xe2x88x92y)1xe2x88x92x/4O3 (PZT, PLT, PLZT depending on the values of x and y), Pb(Mg1/3Nb2/3)O3, KNbO3; hexagonal ABO3 perovskite oxides such as ferroelectrics typified by LiNbO3 and LiTaO3; tungsten bronze oxides such as SrxBa1xe2x88x92xNb2O6 and PbxBa1xe2x88x92xNb2O6; and Bi4Ti3O12, Pb2KNb5O15, K3Li2Nb5O15, and other substitutive dielectrics. The film thickness of the thin-film optical waveguide is usually set from 0.1 xcexcm to 10 xcexcm though it may be properly set depending on purposes.
When a clad layer is used, it can be made of the same material as that of a buffer layer. Specifically, materials satisfying the following conditions are selected as the material of the clad layer: the clad layer has a smaller refractive index than thin-film optical waveguide materials; the ratio of the relative dielectric constant of the clad layer and that of the optical waveguide is 0.002 or more, preferably the ratio of the relative dielectric constant of the clad layer and that of the optical waveguide is 0.006 or more; and the relative dielectric constant of the clad layer is 8 or more. Clad layer materials need not necessarily hold an epitaxial relationship with optical waveguide materials; polycrystalline thin films may be used. However, to obtain a uniform interface, the clad layer materials must be able to hold an epitaxial relationship with optical waveguide materials. As a condition that the epitaxial relationship can be held, it is desirable that clad layer materials are similar to thin-film optical waveguide materials in crystal structure, and the difference of granting constants is 10% or less. However, this condition need not always be observed if the epitaxial relationship can be held. Specifically, the following materials can be used: tetragonal, orthorhombic, or pseudo-cubic ABO3 perovskite oxides such as SrTiO3, BaTiO3, (Sr1xe2x88x92xBax)TiO3 (0 less than x less than 100), PbTiO3, Pb1xe2x88x92xLax(ZryTi1xe2x88x92y)1xe2x88x92x/4O3 (0 less than x less than 30, 0 less than y less than 100, PZT, PLT, PLZT depending on the values of x and y), Pb(Mg1/3Nb2/3)O3, KNbO3; hexagonal ABO3 perovskite oxides such as ferroelectrics typified by LiNbO3 and LiTaO3; tungsten bronze oxides such as SrxBa1xe2x88x92xNb2O6 and PbxBa1xe2x88x92xNb2O6; and Bi4Ti3O12, Pb2KNb5O15, K3Li2Nb5O15, ZnO, and other substitutive dielectrics. It is effective that the ratio of the film thickness of the clad layer and that of the optical waveguide is 0.1 or more, preferably 0.5 or more, and the film thickness of the clad layer is 10 nm or more.
As the electrode for controlling a light beam within the optical waveguide, it is possible to use metals or alloys such as Al, Ti, Cr, Ni, Cu, Pd, Ag, In, Sn, Ta, W, Ir, Pt, and Au, and transparent oxide electrodes having a smaller refractive index than the optical waveguide, such as ITO and Al-doped ZnO. When a clad layer having a smaller refractive index than the optical waveguide is provided between the optical waveguide and an upper electrode, although the upper electrode may be made of any material, it is desirable to use a transparent oxide electrode such as ITO and Al-doped ZnO to prevent an increase in a driving voltage. The electrode for controlling a light beam within the optical waveguide can be formed in a triangular shape and can form a prism area that differs from the surrounding area in refractive index upon application of a voltage. When the optical waveguide has a polarization domain inversion area of prism shape, by applying a voltage to the electrode for controlling a light beam within the optical waveguide, refractive indexes can be made different between the polarization domain inversion area of prism shape and other areas.
As the material of the emission prism, materials having a higher refractive index than the optical waveguide can be used in accordance with on optical waveguide materials and light wavelength. In many cases, rutile (TiO2), GaP, GaAs, or refractive index glass can be used. The base angle of the prism or the angle of an oblique face with respect to the base can be set properly depending on the refractive index of the optical waveguide, a refractive index change of the optical waveguide by electro-optic effect, and the refractive index of the prism.
The transparent upper electrode for controlling an emitted light beam can be made of metals or alloys such as Al, Ti, Cr, Ni, Cu, Pd, Ag, In, Sn, Ta, W, Ir, Pt, and Au that have a smaller refractive index than the optical waveguide. An electrode made thinner, without changing transparency, can be used as the transparent electrode. Also, as the material of the transparent electrode, oxides such as ITO and Al-doped ZnO that have a smaller refractive index than the optical waveguide can be used. However, it is desirable to use transparent oxide electrodes such as ITO and Al-doped ZnO. The transparent electrode for controlling an emitted light beam is disposed between the emission prism and the optical waveguide, and can, by applying a voltage between the transparent electrode for an emitted light beam and the lower electrode, generate different refractive indexes between an area surrounded by the emission prism and the single-crystal substrate or single-crystal thin film and other areas.
A thin-film lens can be provided in the thin-film optical waveguide for the purpose of control of an incident light beam or other purposes. With the effective refractive index of a lens part being larger than that of the thin-film optical waveguide, a circular or pupillary convex lens of the mode index system, a Fresnel lens, or a lens of the grating system can be provided. For example, after fabricating a lenticular ferroelectric thin film that has a larger refractive index than a thin-film optical waveguide, a thin-film lens can be formed by fabricating the thin-film optical waveguide on top of the ferroelectric thin film.
The clad layer, thin-film optical waveguide, buffer layer, and lens layer are fabricated by the solid phase epitaxy of a thin film fabricated by wet processes such as the vapor growth method, sol-gel method, or MOD method, or dry process selected from electron beam evaporation, flash evaporation, ion plating, Rf-magnetron sputtering, ion beam sputtering, laser abrasion, MBE, CVD, plasma CVD, MOCVD, and the like. The most effective method is to subject the buffer layer, thin-film optical waveguide, and clad layer to solid phase epitaxial growth by applying, to a substrate, solution of a metalloorganic compound such as metal alkoxide and organic metal salt by wet processes such as the sol-gel method or the MOD method and further baking the substrate. The solid phase epitaxial growth method is low in facility costs and excellent in uniformity within a substrate compared with various vapor growth methods. In addition, the solid phase epitaxial growth method enables easy and reproducible control of refractive indexes important to the structure control of the buffer layer, thin-film optical waveguide, and clad layer simply by mixing the composition of metaloorganic compound precursors in accordance with a thin-film composition having a refractive index necessary for the buffer layer, thin-film optical waveguide, and clad layer, and also allows the growth of the buffer layer, thin-film optical waveguide, and clad layer having low light propagation loss.